There is shown in FIG. 1 a microwave oven including a housing 1, a power supply unit 2 having a high voltage transformer (not shown) and a high voltage condenser (not shown), a cylindrical magnetron 10 for generating a microwave frequency energy and a cooking chamber 3 for containing food therein. As shown in FIG. 2, the magnetron 10 is a cylindrical bi-pole vacuum tube and typically includes a cathode 11 arranged at the center thereof, a pair of magnetes 12a, 12b disposed thereabove and therebeneath respectively, an anode 13 arranged around the cathode 11 and an antenna 14 connected to the anode 13.
When an operating voltage of, e.g., 4 KV, is applied to an input terminal 15 from the power supply unit 2, the cathode 11 is heated to emit electrons. The emitted electrons are received by the anode 13.
The magnets 12a, 12b generate magnetic fluxes which are, in turn, guided by guide members 16a, 16b to pass through a cavity 17 which is defined between the cathode 11 and the anode 13. The electrons emitted from the cathode 11 are first deviated by a magnetic field formed in the cavity 17 so that they revolve between the cathode 11 and the anode 13 prior to traveling to the anode 13 and being received thereat.
Revolving of the electrons between the cathode 11 and the anode 13 results in a resonant circuit being constructed in the anode 13, the resonant circuit generating microwaves to be emitted through the antenna 14. The emitted microwaves are guided to the cooking chamber 3 by a waveguide 5 and then spread in the cooking chamber 3 by a stirrer 6. The spread microwaves are incident on food contained in the cooking chamber 3 so that cooking of the food can be carried out.
In such a microwave oven, since the motion of electrons is controlled by the combined force of both electric and magnetic fields, a plurality of magnets are required, which, in turn, makes the microwave oven structurally complicated. Further, since the microwave frequency energy generating apparatus employed in the conventional microwave oven is of a bi-pole type, it is impossible to control the output of the microwave frequency energy.